Press-Fit Ground
Rotor-Fit Shafts

A rotor-fit shaft is a motor shaft specifically dimensioned for the interference press fit of a rotor lamination stack or permanent magnet rotor. The rotor fit zone uses p5 or r5 tolerance (slight interference) to hold the rotor stack without movement under motor torque. Concentricity between the rotor fit and the bearing journals must be within 0.005 mm — otherwise the assembled rotor is eccentric, causing vibration and magnetic cogging variation.

p5 or r5 shaft tolerance for rotor stack press fit — these give an interference of approximately 0.005–0.020 mm on diameter (depending on shaft diameter). The resulting hoop stress in the rotor bore must be checked not to exceed the rotor lamination yield strength. For magnets bonded rather than pressed: h6 shaft tolerance with adhesive bonding. For wound rotor or slip-ring designs: the fit depends on the rotor body material — HXC engineers advise based on your motor design data.

All rotor fit and bearing seat diameters are ground in a single 5-axis turning-milling setup without rechucking. This eliminates the setup error that would occur in a two-operation approach, achieving concentricity ≤ 0.005 mm TIR between rotor fit OD and bearing journals. This is the critical tolerance for assembled motor runout — if the rotor fit and bearing seats are not concentric, the assembled motor will vibrate at the fundamental frequency, generate magnetic noise, and reduce bearing life.

Dynamic balance removes residual unbalance from the finished rotor shaft, typically performed after hardening and grinding. For motor shafts: G1.0 is standard for industrial servo motors; G0.4 for precision spindle motors and high-speed motors above 10,000 RPM. Balance is performed on a two-plane balancing machine, with material removed by drilling small holes at the balance correction planes. HXC provides the ISO 1940-1 balance report with every balanced shaft.

Prototype (1–5 pcs, hardened + ground + balanced): 5–8 days. Small batch (10–50 pcs): 8–12 days. Production (100–1,000 pcs): 10–16 days. Stainless or titanium: add 2–4 days. G0.4 balance grade: add 1–2 days. Rush production available.

42CrMo4 (induction hardened) for BLDC and servo motor rotors — the most common choice. 20CrMnTi (carburized) for EV traction motors and high-cycle rotor assemblies. GCr15 (SUJ2 / 100Cr6) through-hardened for high-speed spindle motors. SUS440C hardenable stainless for clean-room and vacuum-chamber motors. SUS303/316L for pharma micro-pumps and lab automation. Al 7075 hard-anodized for drone and lightweight motors.

Rotor fit zone: Ra 0.4 µm (standard for press fit). Bearing journals: Ra 0.2 µm. Output shaft seal seats: Ra 0.1 µm mirror finish if required. All measured by contact profilometer and reported on the inspection certificate.

Yes — keyways, splines, threads, cross-holes, D-flats, and tapers can all be added on the output end in the same 5-axis setup. Concentricity between the rotor fit zone, bearing journals, and shaft-end output features stays within 0.003 mm TIR — single-setup machining is what enables the full feature concentricity stack-up.

The rotor stack is pressed onto the shaft using an arbor press or hydraulic press with the shaft supported in a V-block fixture to prevent bending. For magnetised PM rotors, a non-magnetic press tool is used. For shrink-fit assembly (an alternative to press fit), the rotor stack is heated to 80–120 °C and the shaft is cooled, allowing it to slide together with zero force; the joint then becomes interference-fit at room temperature. HXC can supply rotor fit zone with chamfered lead-in (typically 1×30°) to ease press-on starting.